1. Field of the Invention
The present invention relates to a projection optical system which is configured to project light from an image forming element to a plane to be projected, and relates to an image display device including the projection optical system.
2. Description of the Related Art
JP 2006-235516 A discloses a projection display device which includes a projection optical system having a lens system and one concave mirror. The concave mirror is arranged below the optical axis of the lens system in a largely bulged state. JP 2007-225776 A discloses a back projection type (rear projection type) image projection device, in which projection light from a final mirror crosses an optical path between a lens system and a mirror system, and a screen for the image to be projected on (a screen) is arranged almost in parallel with an optical axis of the lens system, and thinness of the entire device including the screen is improved.
In the projection optical system disclosed in JP 2006-235516 A, the concave mirror bulges largely below the optical axis of the lens system, therefore if the projection optical system is applied to a front projection type device, when constituting a projection device (a projector main body excluding a screen), since a bottom of a case is defined based on a lower end of the concave mirror, an increase in a size of the projection device is necessary, and thus problems arise such that a height of the device is great, and the gravity center of the device is high due to the optical axis being located on an upper side in the case of the device, and instability to an installation surface (a setting plane) of the projector main body is unavoidable.
Similar problems exist in projection devices disclosed in JP 2004-309765 A and JP 2007-079524 A.
In the projection device disclosed in JP 2007-225776 A, with a purpose of reducing a width in a thickness direction of the screen of the entire device which includes a projection optical system unit PU and the screen, a thin device is achieved.
However, due to the projection optical system unit PU being optimized as a rear projector, it can not be applied to a front projector directly.
When the projection optical system unit PU is applied to the front projector, such a structure is available in which light flux is folded in the projection optical system by such as a folding back mirror, to make it possible to view a projection image from a viewing side; however, for the projector main body in which the projection optical system other than the screen is housed in the case, compactness is inhibited, and the cost rise is unavoidable.